Tank Size?

[Teller]

Without getting too complicated how do you determine the size of the tank you need. I want to start doing deeper dives and I am looking at getting steel tank. Im about 5'7 150lbs and I dont want to get something that is going to be overkill. I was thinking a HP100. How do you choose the right tank for you?

 

[iztok]

Rent different tanks or borrow from friendly divers and see what works for you.

 

[Bubbletrubble]

Figure out the kind of dives that you want to do (depth, time considerations).
Learn how to calculate how much gas you need to do those dives. To do this, read and understand Bob Bailey's (NWGratefulDiver) Gas Management essay.

Once you've done this, figure out which tanks have physical dimensions that trim out best for you (per iztok's recommendations). If you have multiple tanks in your dive shed, it can be convenient to own tanks with the same diameter. This means that you won't have to keep adjusting your tank cambands.

The HP100 is a great tank. Dive it. You'll like it. But make sure that it's capacity is a good match for the dives that you want to do (RE: gas management).

 

[rjack321]

Overkill isn't the end of the world. Can't make more gas on the dive.

The hp100 is a great tank but its not huge and for most people its realistic max functional depth is around 90ft. If you are planning on 120ft dives down the road you'll want more gas. Gas management calculations (like Bob's) will elaborate on this but cf carried = depth in feet is a pretty good rule of thumb to help keep adequate reserves.

 

[Teller]

Overkill isn't the end of the world. Can't make more gas on the dive.

The hp100 is a great tank but its not huge and for most people its realistic max functional depth is around 90ft. If you are planning on 120ft dives down the road you'll want more gas. Gas management calculations (like Bob's) will elaborate on this but cf carried = depth in feet is a pretty good rule of thumb to help keep adequate reserves.

Does this mean that with the right training I should be able to take two HP100's down to 200 feet?

 

[DA Aquamaster]

Does this mean that with the right training I should be able to take two HP100's down to 200 feet?

It depends on your SAC, bottom time, use of stage, deco gasses, etc, but yes 200' is very doable on a set of double 100s with bottom times in the 25 min range with at least a 1/3rd reserve. My double 100's have 18/35 in them right now for exactly that type of profile.

 

[David_57]

Does this mean that with the right training I should be able to take two HP100's down to 200 feet?

Waaaaay more complicated than that my friend things like SAC rate time, at depth, deco obligations, Gas mix, etc come into play.

 

[scubadiver888]

Here is how I determined the size of tank I needed:

- What is available in your area?
No point in select a tank you cannot find.

- Do you want aluminum or steel?
Aluminum oxidization is less of a concern than steel rusting. I'm a fresh water diver so rusting is not as big a concern. A steel cylinder is going to be more negatively buoyant. So I can remove weight from my belt. If I had very little weight, the whole setup might make me too heavy. Since I dive with a 7mm wetsuit this was not a concern. I had plenty of weight to move off my belt. If you dive with less than 6 pounds it might be an issue.

- What are the buoyancy characteristics?
I narrowed my selection down to HP steel cylinders. All cylinders become lighter as you breath down the air. HP steels change from 6 to 10 pounds. Diving with an HP130 means I need to be around 10 pounds too heavy at the start of the dive (otherwise I'll be light at the end of the dive). You normally never breath down all your air but in an emergency situation (free flow) you might. You really don't want to become positively buoyant during an emergency situation.

Everything from HP100 to HP130 are about the same change +/- a pound.

- What are the weight differences out of the water?
The HP100 is 33 pounds and the HP130 is 43 pounds. If you don't ever see yourself needing the extra 30 cu.ft. of air, do you really want to have an extra 10 pounds of weight? Do you do a lot of shore dives? Is it a long walk to the entry point? Do you have a bad back or knees?

- How tall are you? How does an AL80 work for you?
If you find an AL80 long, it is around 26 inches. An HP100, HP119 or HP130 are shorter. An HP120 is 28 inches long. Personally, at 5'7" I think an HP120 would be too long for you. If you find an AL80 a little long then HP100 or HP119 are only 24 inches long.

- What kind of diving do I do? What is my SAC rate?
If I was diving 30 foot reefs and a SAC rate of 0.5 then an HP130 would last me 130 minutes. NDL is 205 for 35 feet. Bigger is better.

If I was diving wrecks at 100 feet and a SAC rate of 0.7 then an HP100 would last me over 30 minutes. NDL is 20 minutes. So if I use rule of thirds this is fine. If you use Rock Bottom Gas Management, you might want to consider the HP130. It would last you another 8 minutes at depth.

- Is there a bargain?
I walked into a dive shop and they had one HP119 on sale. Floor model, one year past birth date. Their pricing was very competitive and they were giving 35% off with current visual, o2 clean and filled. I had decided on an HP100 but this thing was 123 cu.ft. of air, same length as HP100 (a trim concern for me), similar buoyancy characteristics, 9 pounds heavier on land BUT less than the price of an HP100 out of hydro.

- Finally, would you be switching to doubles some day? Would you consider using a pony bottle when more air is needed emergency situations?

 

[rjack321]

Does this mean that with the right training I should be able to take two HP100's down to 200 feet?

You wanted the simple version. The right training includes help figuring this out and experience too. But simplistically, yes 200cf is "enough" gas for short 200ft dives assuming you've brought the right deco gases and your stressed consumption isn't attrocious.

On the opposite end, an AL80 really isn't "enough" gas for 90ft dives.

 

[c5diver]

i tried an hp100 it was just to heavy for me it made me back heavy causing me to roll and in the end i really didn't need that much gas. if you have a good sac rate an 80 is plenty i can dive an 80 to 100fsw to the ndl the accend and continue the dive and still have plenty of back gas left. but my avg sac is low .31 to .35